Boot 2 Root competitions refer to CTF challenges where the player starts with a new machine on the same network which they have no credentials to log into.
The most popular Boot2Root challenges are hosted on VulnHub and Hack the Box. Vulnhub is a repository of Boot 2 Root challenges where people can upload the challenges they've made as VM images. These require players to have their own virtualization software to download, install, and run the VM. For a tutorial on this process, go to VulnHub. Hack the box is another popular choice because all challenge are hosted by hack the box and accessible through a vpn connection provided by the platform.